In a radio communication systems, information (for example speech, image information, video information, SMS (Short Message Service) or other data) is transmitted using electromagnetic waves over a radio interface between transmitting and receiver stations (base station or subscriber station). In such cases, the electromagnetic waves are propagated using carrier frequencies lying within the frequency band provided for the relevant system. For the GSM (Global System for Mobile Communication) mobile radio system which has been introduced, frequencies at 900, 1800 und 1900 MHz were used. For future mobile radio systems with CDMA or TD/CDMA procedures, such as UMTS (Universal Mobile Telecommunication System) or other third-generation systems, there is provision for frequencies in the frequency band of around 2000 MHz.
The access from stations to the shared transmission medium is regulated in these radio communication systems by Multiple Access (MA). With these multiple accesses, the transmission medium can be subdivided between the stations in the time area (Time Division Multiple Access, TDMA), in the frequency area (Frequency Division Multiple Access, FDMA), in the code area (Code Division Multiple Access, CDMA) or in the space area (Space Division Multiple Access, SDMA). In this case, the transmission medium (with GSM (Global System for Mobile Communications), TETRA (Terrestrial Trunked Radio), DECT (Digital Enhanced Cordless Telecommunication), UMTS (Universal Mobile Telecommunication System) for example) is frequently subdivided in the frequency and/or time channels in accordance with the radio interface. These channels are generally referred to as transmission channels or radio channels. For systems where coordination is decentralized, measurements are used to decide on the usability of these transmission channels. In accordance with the radio radiation (i.e., depending on the radio field attenuation), re-use of these transmission channels at an appropriate spacing is possible.
For radio transmission between a transmit station and at least one receiver station of a radio transmission system, interference occurrences now arise as a result of the frequency selectivity of the transmission channels, such interference being known as intersymbol interference and Multiple Access interference. The greater the transmission bandwidth of the transmission channel, the more these interferences distort the transmit signals.
Conventionally, the transmit signals are generated at the transmit station without taking account of the effective radio channels. The interference occurrences then arising are rectified in a second step, at least approximately by the appropriate matched and generally very expensive methods for detecting the transmitted data at the receiver stations.
Radio communication systems with at least one transmit station (Transmit station AP or base station) and at least two receiver stations (Receiver station MT) are known, with the transmit station (AP) and the receiver stations (MT) being connected to one another over a radio communications interface. Here, the transmit station features a transmit antenna with KB antenna elements (with KB≧1) and the receiver stations each feature a transmit antenna with KM antenna elements (with KM≧1). They communicate by MIMO (Multiple Input-Multiple Output) transmission.
Radio transmission devices with at least one transmit station having a number of transmit elements and with at least one receiver station having a number of receive elements are referred to in this document as MIMO systems. Radio transmission between at least one transmit station and at least one receiver station of a MIMO system is subject, as a result of the frequency selectivity of the transmission channels, to interference occurrences which are known as intersymbol interferences and Multiple Access interference. For the purposes of radio transmission from at least one transmit station to the receiver stations in a MIMO there are basically two requirements which need to be fulfilled:                suitable transmit signals are to be generated and propagated by the relevant transmit station for each of the transmit antennas; and        the data which is of interest in each case is to be detected by each of the receiver stations by suitable processing of the receive signals of all receive antennas.        
In recent years, alternative concepts, such as Joint Transmission or Joint Predistortion have been investigated, which, by taking account of the effective transmission channels, eliminate the interference occurrences completely, to a large extent or at least partly at the point at which the transmit signals are being generated at the transmit station. See, for example:                M. Meurer, P. W. Baier, T. Weber, Y. Lu, A. Papathanassiou, “Joint Transmission, an advantageous downlink concept for CDMA mobile radio system using time division duplexing”, IEE Electronics Letters, Bd. 36, 2000, S. 900-901 [1] and        P. W. Baier, M. Meurer, T. Weber, H. Tröger, “Joint Transmission (JT), an alternative rationale for the downlink of time division CDMA using multi-element transmit antennas”, Proc. IEEE 7th International Symposium on Spread Spectrum Techniques & Applications (ISSSTA'2000), Parsippany/N.J., 2000, S. 1-5 [2].The cited documents present a Joint Transmission (JT) transmission method, especially for downlink mobile radio systems from the base station to the subscriber stations, which allows simultaneous provision to a number of subscribers. The transmit signals propagated by the transmit antennas of the base station or transmit station (AP) are generated in a common process in this case and optimized with respect to the transmit energy to be used.        
In Joint Transmission systems with at least one transmit station having at least one transmit antenna and with at least one receiver station having at least one receive antenna, the linear receive-side signal processing, referred to below as demodulation, is described by receiver station-specific demodulator matrices [2].
In conventional Joint-Transmission systems [2], the subscriber-specific demodulation matrices are defined by fixed signatures; e.g., CDMA codes. This process is particularly determined due to the fact that no information about the space and time transmission characteristics of the mobile radio channels operating between transmit stations and receiver stations is included in the design of the subscriber-specific demodulation matrices.
Similar to the process used in the Joint Transmission (JT) method, such transmit signals also may be generated when receiver stations with a number of receive antennas are employed, by using                information about the effective radio channels and        information about the receive-side-specific processing methods defined a priori for detection,which, theoretically, perfectly eliminates the interference occurrences discussed at the point of transmission.        
The present invention is, thus, directed toward a method and an improved transmit device which, for the effective transmission channels, takes account both of a minimization of the transmit power and also of further quality criteria, such as a directional characteristic of the transmit signal for example.